Bridge: pin the Objective-C API inventory and build gate

Parent: #138 Depends on: #135 and #137

Pin the declared macOS Objective-C JavaScriptCore surface before implementation so bridge completeness is exact and auditable.

Acceptance:

• Check in the macOS 27.0 / SDK build 26A5368g JSContext.h, JSValue.h, JSVirtualMachine.h, JSManagedValue.h, and JSExport.h target inventory with source hashes.
• Inventory every targeted Objective-C class, protocol, initializer, method, property, category, macro, and availability boundary with no hidden declarations.
• Add an audit tool that rejects checked-in header drift, duplicate/missing entries, invalid status values, and unlinked unfinished entries.
• Check in compatible Objective-C umbrella headers and explicit platform/version gates; unsupported hosts or SDK revisions fail clearly.
• Add a macOS-only Objective-C compile-link smoke target without imposing Objective-C/Foundation on non-macOS C/Zig consumers.
• Document the declared target and reproduction commands in README and API docs.

Foundation pushed in two reviewable chunks: 11ffd37 pins normalized installable copies plus raw/normalized SDK hashes, 11 containers, and 108 selector/signature-level declarations; 9ef5672 adds the Apple-only umbrella and ARC/blocks compile gate.

Passing gates:

• zig build objc-api-audit
• zig build test-objc-api-headers
• live SDK comparison against macOS 27.0 build 26A5368g
• unchanged 117/117 C API audit and real C/C++ hosts

The compile-link-runtime checkbox remains open until #158 provides real Objective-C class symbols; the current gate is deliberately named test-objc-api-headers.

Completed through d06e264. The pinned macOS 27.0 / 26A5368g inventory now drives header drift, real ARC/blocks header compilation, and a macOS Objective-C compile-link-runtime host (zig build test-objc-api) without adding Foundation requirements to non-macOS or plain C consumers. Current runtime coverage is tracked separately in #158 at 32 implemented / 76 pending.